kendall



B. VV. KENDLL MONITORING SYSTEM Filed Sept. 5, 1919 2 Sheets-Sheet l W, KENDALL Em. E923 MONITORING SYSTEM Filed Sep'c. 5, 191.9 2 Sheets-Sheet 2' /Mvenfof/f OIIQIIL Patented Dec. 8, 1925.

. UNTED S'EA'E BURTON W. KENDALL, OF NEW YORK,

N. Y.. ASSIGNOR T0 WESTERN ELECTRIC COIEPANY, INCORPORATED, OF NEW YORK, N. Y.. A CORORTION OF NEW YORK.

MONITORING Application filed September To all 107mm t may concern:

Be it known that I, BURTON lV. KENDALL, a citizen of the United States, residing at New York, in the county of New York, State of New York, have invented certain new and useful Improvements in a h/ionitoring System, of which the following is a full, clear, concise, and exact description.

This invention relates to signaling, and more particularly to transmittingI and receiving` circuits having' flexibility whereby they may be readily a'djusted for differentsignals. The invention covers also arrangements for connecting' said circuits to a line or to a repeater, and such other features as will hev described in the followingl specification and included in the accompanying claims.

A more specific aspect of the invention relates t-o repeat-er circuits for lmultiplex signaling` employing carrier currents on wires, und particularly to a means for monitoring telegraphic and telephonic messages at the rcpeater station. By means of the invention messages that are being repeated in either direction may he received at the repeating station, and messages may also be sent over either line from the repeating station through any channel of either line.

The invention will be better un'derstood from the drawings in which Fig. 1 is a schematic diagram of circuits for monitoring' telegraphic messages according to the invcntion; and Fig. 2 shows means for monitoring telephonic messages.

Main lines 1, 2 and 1, 2 may be carrier current transmission lines for transmitting several current waves dilfering in frequency and forming communication channels through which messages are conveyed in the form of modulations of the respective carrier Waves. Each line 1, 2 terminates at the repeater station shown in means for coupling to the input of one repeater for repeating into the other line, and to the output of another repeater for receiving a message being repeated from the other line. Since all this apparatus is entirely symmetrical, a double series of reference characters is used, and the following description may apply equally well to one repoater and line or to the other SYSTEM.

5, 1919. Serial No. 321,900.

repeater and line. The path of the repeatcd message will first be traced through from one line 1, 2 to the other.

Cnrrents incoming from line 1, 2 pass through the coils 3, 4, 5 and 6, and in lpart through the coil 1'? bridged across the line. By means of the coupling` between coil 17 and a eoil 18, the incomingl currents are imprcssed across a resistance 19 forming a potentiometcr for the input terminals of the fi *st repeating amplifier. By means of the sliding; contact 21, the voltage impressed on the input of the first amp-lifier 20 may be regulated. Amplifier 20 is shown as of the thermionic type, comprising an enclosure in which are a hot cathode or filament 22, a gri'd 23 and a plate or anode 24, or their cquivalents. The filament is supplied with heatingl current from battery 25, and plate potential is supplied from battery 26 connected to the filament and to the plate 24. through a choke impedance 27. The normal potential of the grid 23 with respect to the filament'is fixed by a. battery 28. A condenser 29 affords a low impedance path for the Variation currents, and acts as a blocking condenser to prev'ent fiow of direct current from battery 26 through co-il 27 an'd the grid resistance 32 of the second-stage amplifier, Resistance 32 permits the normal potential of the grid 31 of the second-stage amplifier 30 to be fixed by battery 33, relative to the filament, but is vsufiiciently large not to act as a shunt for the out-put waves from amplifier 20. Amplifier 30 may, be of the same general type as amplifier 20, and act in a similar manner to impress the transmitted Waves in amplified form through condenser 84 upon coils 11 and 12. Each line 1, 2 is provided with a net having electrical characteristics equivalentto those of the line. Coils 3 and 4 are also similar to each other and to coils 9 and 10, and coils 5, G, 7 and 8 are likewise similar to one another. By this arrangement the repeated waves from output windings 1.1 and 12 are fed into the outgoing` line without aifecting the input of the other side of the two-way repeater. Grounded shields 85, 35 are provided to protect against disturbances due to capacity effects between the coil windings.

Thus far the description has been concerned merely with the repeating of the high-frequency modulated carrier currents between the main lines 1, 2 and 1, 2. The provisions for monitoring the rcpeatcd messages will now be described.

Coils 8 and 9 are for the purpose of coupling to the coils 13 and 14, which latter coils are included` in circuit with a coil 15. Coils 4 and 10 may be included in the coupling for coils 13 and 14. Coupled to coil 15 is a coilv 16 in a circuit leading in one case to jack J1 and in the other case to jack J 2. A load is normall present on the transformer 15, 16 in the orm of an impedance 36v across the' terminals of coil 16. A plug P is provided for inserting into either jack J1 or .12 to couple the telegraph monitoring ap aratus to one or the other line through'tie corresponding coil 15, 16. The telegraph monitoring appa-ratus comprises tuning circuits connected to plug P for tuning to a desired channel frequency, a generator and transmitter for sending over the dcsired channel, a receiyer circuit for receiving on any channel, and a switch for throwing either` the transmitting circuit or the. receiving eircuit into communication with the tuning circuits and plug P.

The transmitting Circuit comprises a generator with a key for controlling its output and an amplifier in its output circuit. A generator 40 of the thermionic-relay type is shown having a coupling between its output and input circuits. It eomprises an enclosure in which may be a hot cathode, or other source of electrons, shown as filament 41, a plate or anode 42, and a grid or impedance-controlling element 43. Hea-ting eurrent for the filament is supplied from battery 44, through a regulating resistance. Plate potential is furnished by battery connected on one side to the filament and on the other side to the plate through a suitable impedan'ce 46. A space current is thus caused to flow between the heated filament 41 and the plate 42, and any variations in this current find a path through the condenser/ 47 of large ca acity, and in part, through inductanee 48 lack to the filament. In shunt to the inductance 48 are (when contact 49 is closed) an inductance 50 and a variable capacity 51. This combination of inductance and capacity 48, 50 and 51 may be tuned by adjusting condenser 51. The grid 43 is connected to this tuned circuit so that if there are oscillations in the tuned eircuit, the potential of the grid Will be periodically varied accordingly, and these variations of grid potential will cause corresponding current Variation-s to flow in the plate circuit through condenser 47l A portion of this output current will be fed back to coil 48, as describcd, and the remaining portion will flow through the 'power-output Luc-1,807

branch including the potentiometer resistance 52. The energy fed back to the input circuit of the generator will be sutiicient to cause the continued generation of oscillations, the frequency of which, if contact 49 is closed, may be controlled by adjustable condenser 51. By means of the sliding contact on the potentiometer resistance 52, the maximum potential to be impressed upon the grid 58 of the amplifier by the output oscillations from the generator may be chosen. The amplifier may be similar in construction to the thermionic element of generator 40, and may comprise an enclosure with a filament 54 heated by current from battery 44, a plate 56, and a grid 53 already menti'oned. 7 hen the transmitting circuits are being used, a switch is closed on the left side, marked T, and the output Circuit of amplifier then may be trace'd from plate 56 through the upper side of switch S through coil 57 and condenser 58, in parallel., through the lower side of switch S, plate battery 45 to filament 54 and through the space portion to plate 56 again. The amplified oscillations from the generator 40 are thus impressed on coil 57 when the switch S is thrown to transmitting position. If it is desired to transmit over the line 1, 2 at the left, the plug P is inserted in jack Jl. The Springs of the jack are spread so that the mpedance 36 is removed from across the jack and the jack springs ar;J now connected through the tip and ring` eontacts of plug P to the series resonant Circuit comprising inductance 59 and condenser 60. By means of condenser 60, the circuit includinf' coil 16 and elements 59 and 60 may be tuned to the frequency of the channel over which it is desired to transmit. The coupling between coils 57 and 59 may also be adjusted. The condenser 58 permits the circuit 57, 58 to be tuned to the frequency being transmitted. By means of variable condenser 51, the frequency of the generated wave may be adjusted to the proper ehannelfrequency. W'ith these adjustments made and the switch S closed to the transmitting side, a continuous Wave of the selected carrier frequency will be impressed through coils 57, 59 and 16, 15 upon coils 18 and 14, as long as contact 49 is closed. Contact 49 may be controlled in any suitable manner and is shown as operatcd by the armature of the relay 61, which is in turn Controlled by a circuit including battery 62 and a key K, so that with key K closed, contact 49 is closed. If now the key is opened, contact 49 is opcned, and the condenser 51 is reinoved from the tuned circuit controlling the input of the generator. The shunt capacity of the coils 48 and 50 is comparatively very small and the. frequency of the generator immediately mounts up to a rclatively very high value, as soon as condenser 51 is rcmoved from the Circuit.

These high-frequency oscillations are prevented from reaching the coils 13 and 14 by the combination of tuned circuits including the inductances 57, 59 and condensers 58 and 60. The result is that with' key K closed a continuous carrier Wave is impressed on the line 1, 2, through coils 13, 14, associated with coils 3, 4, 9 and 10, and that with key K open, no current is transmitted to the line 1, 2. By virtue of the balance of the line and the net and the coils 3.to 10 inclusive, as to the terminals of coil 17, as previously explained, the current thus transmitted causes no d'isturb'ance to the input coil 17 of therepeater. If the plug P had been inserted into jack J2, the transmission could be effected into line l, 2 at the right. Condensers 51, 58 and G0, together with the inductances, may have such range of adjust'- ment as to permit the monitoring set to communicate over any channel. lVhen plug P is removed from the jack, impedance 36 is substituted across the Springs of the jack in place of the circuit connected to the plug, in order better to maintain the existing' balance of the line and net work.

The receiving side of the monitoring set is shown as comprising a two-stage amplifying and a detecting circuit controlling a relay which may in turn control a sounder or other receiver or recorder. In the form shown, the first two stages and are thermionic amplifiers, and the third or detecting stage is a thermionic detector. Amplifiers 65 and 75 are similar and are shown as each comprising an evacuated enclosure, containing a hot cathode or filament 66, 67, respectively, a plate or anode 68, 69, respesctively, and a grid or impedance controlling` element 70, 71, respectirely. Heating current is furnished to filaments G6 and 67 from battery 71', through regulating resistance 7 2, and plate potential is supplied from battery 73 connected to plate G8 through choke impedances 7a and 76, and to plate 69 through choke impedance 77, which oppose the passage through them of variable currents in the output circuits of the respective. amplifiers. The variation currents are thus caused to pass through condensers 78 and 79, respectively. A polarizing battery 81 is shown for fixing the normal potential of grid 71 with respect to iis fil'ament, and is connected between the grid 71 and the filament 67 through a suitably high resistance 82. A similar battery 81' is shown for fixing the potential of grid 70, The detector SO is shown as comprising an evacuated enclosure containing the filamcnt 83, heated by current from battery 71' through a regulating resistance,`a grid 84 and a plate 85. The external circnit between the plate. 85 and filament 83 includes a relay 86 and plate battery 7 3, and in shunt of this path is the small condenser 87. The normal potential of grid 8-1 may be fixed by polarizing battery 88 connected to grid 8-1- and through resistance 89 to filament 83.

VVith the plug P inserted in the desired jack Jl or .72, and with the selective circuits 57, 58, 59, 60 tuned to pass the frequency of the channel from which it is desired to receive, switch S is thrown to the receiving position indicated at R and the inconiing' waves are impressed upon the amplifying and detecting circuits. The output ctu'rent from the detcctor, due to the inductance of the winding of relay 8G and the capacity of the shunt-ing condenser 87, is practically a v Constant current of a duration corresponding to the length of the wave train received. The length of the wave train may depend upon the time of depression of a key at the distant transmitting station similar, for instance, in its action to key n. Thus, the relay 88 pnlls up and releases its armature in accordance with the actuation of the dist-aut transmi'tting key or other transmitting arrangeinent. Contacts for the relay armature are shown adapted to control a suitable local circuit in which may be a sounder or recorder or other receiver. Either or both relay contacts may be used, but a portion of a local c-ircnt including the back contact of the relay is illustrated.

In F 2 a provision for monitoring telephone messages is shown. For selectively transmitting voice modulated carrier waves, the simple tuned circuitconnected to the plug' P of Fig. 1 is insuiiicient since the voice-inodulated wave comprises a band of frequencies, all of which must be transmitted with substantially uniform eliiciency to avoid distortion. A series of band iilters F, F1, F2, etc., are provided, each so proportioned asto transmit a different carrier wave and the range of frequencies comprised in the wave when it is modulated in accordance with speech. A plug P' is provided on each end of each of said filters and a jack J is in permanent connection with the switch S. lVlien it is desired to communicate telephonically over any channel of either line 1, 2, the filter designed for that channel is selected and its plugs P' are inserted, one in jack J3 and the other in jack J1 or J2 depending on the line to be used. The switch S is thrown into position T if it is desired to transmit or to position R if it is desired to receive. Assuming itis desired to transmit, the condenser 51 is adjusted so that the generator 40 produces current of the carrier frequency desired, and with the switch S in transmitting position, speech iS directed against the microphone 90, and as' a result of the coupling 91, voice Waves are impressed upon the generator circuits and the carrier wave being produced is modulated in accordance with the voice Waves. The

energy of the generated wave may be controlled by a non-inductive, adjustable 'resistance 93. The proper filter F, F1, etc., for thecarrier wave employed permits the voice modulated wave to be transmitted to line 1, 2 snbstantially without distortion. IVith switch S thrown to the receiving position R, the incoming voice modulatcd wave is conveyed through the connected band filter to the input of the amplifiers G5 and 75, is detected by detector S0 and rendered audi.- ble by .receiver 92.

lVhile, for'the sake of a complete and clear disclosure, elements have been shown and described specifically, it is to be understood that it is innnaterial what particular type of transmitter or generator or receiver or repea-ter is used and the invention is not to be limited to the particular arrangements shown. Furthermore, certain features of the invention, as will appear in the accompanying claiins, are of more gene'al application than for use with repeater, and the invention :is to be limited only by the scope of the a ppenced clain'is.

VVhat is claimed is:

1. A carrier current signaling' system comprising a line having coupling-coil line windings and a. net circuit for balancing said line, a monitoring circuit syn'nnetrically associated with said line windings, and carrier current signal transmitting and receiving apparatus for connection to said circuit.

2. In a signaling system, lines each having coupling-coil line windings and a net circuit for balancing each line, a single circuit for each said line symmetrically associated With said respective line windings, a transmitter and a receiver for said lines, and means for connecting' said transmitter and receiver to any one of'said symmetrically associated circuits to the exclusion of the other circuit.

3. In a signaling system employin currents of different frequencies for di erent transmission Channels, a line having couplirg-coil line windings and a net circuit for balancing the line as to all said frequencies, a circuit symmetrically associated with said line windings, transmitting and receiving apparatus for connection to said circuit, and means selective as to said frequencies interposed between said circuit and said transmitting and receiving means.

4-. A carrier current system comprising two line Sections terminating in a repeater station, a. two-way repeater for repeating between Said line Sections simultaneously in both drections, and a monitoring system at said repeater station for transmitting or for receiving carrier currents over either line section simiiltaneously with said two-way repeating.

5. A carrier current repeating system comprising av line circuit having a bridging point and an artiticial circuit in balance with said line as to said bl'idging point, a repeater connected to said line at said bridging point, and a transmitter for transmitting current to said line and a receiver for receiving current from said line, each arranged for connection in the same. manner with Said line in balance relation as to said bridging point. i

(i. A carrier current repeat-ing system comprising a line circuit having a bridging point and an artificial circuit in balance with said line as to 'said bridging point, a repeater having one Side connected to said line at said bridging point, another repeater having its opposite side connected with said line in balanced relation as to said bridging point, and a transmitter and a receiver each arranged for connection in the same manner with said line in balance relation as to said ln/idging point.

i. In a signal repeatmg system, a line, a circuit for transmitting currents of different frequencies for different transmission cliannels having a bridging point and an artiiicial circuit in balance with said line as to said bridging point for all the frequeucies on said line, avrepeater for said currents connected to said line at said bridging point, a circuit selective as to said frequencies ai'ranged for connection with said line in balanced relation as to said bridging' point, and a transniitter and a receiver airanged for connection with said selective circuit. i

S. A, carrier current signaling system comprising a line having; coupling coil line windings and a. net for balancing said line, a two-way repeater coupled to said line windings for transferring signaling current in both directions between said hne and said repeater, and a sepa'ate circuit symmetrically coupled to said line windings for simultaneously transferring current to or receiving current from said line and two-way repeater.

9. In a carrier-wave signaling system. lines for transmitting each a plurality of carrier Waves differing in frequency, a selective circuit, means for coupling said selective circuit to any one of Said lines and for adjusting it to be selective to a desired said carrier Wave, a transmitter and a receiver for said lines, and means for coupling said transmitter and said receiver through said selective circuit with said desired line.

10. In a carrier-wave signaling system, lines for transmittingl each a plurality of carrier Waves diifering in frequency, a selective circuit, means for coupling said selective circuit to any one of said lines and for adjusting it to be selective to a desired said carrier wave, a transmitter and 'a receiver for said lines, means for couplin` said transinitter and said receiver through said selective circuit with said desired line, and means for adjusting said transmitter to the frequency of a desired said Carrier wave.

ll. In a multiplex signaling system, lines, each having` a plurality of discrete communication channels for the transmission of signals theretln'ough, a selectiye circuit, means for connecting said Circuit to any one of said lines, means in said circuit for selecting any desired communication channel on the desired line, a transmitter and a receiver for said lines, and means for connecting said.

transmitter and said receiver through said selective circuit to the-desired line.

12. In a multiplex signaling system, lines, each having a plurality of discrete communication Channels for the transmission of signals therethrough, a selective circuit, means for connecting said Circuit to any one of said lines, means in said circuit for selecting any desired communication channel on the desired line, a transmitter and a receiver for said lines, means for connecting said transmitter and said receiver through said selective circuit to the desired line, and means for adjusting said transmitter for signaling through the selected channel of the desired line.

13. In a carrier-wave transmission system, two line sections, means 'for transmitting a plurality of carrier currents forming a plurality of communication Channels simultaneously on said line sections, two-way repeating` means between said line sections for all said Channels, and monitoring' means for receiving or for transmittinp; over either line section through any desired said channel.

14. In a carrier-wave transmission system, two line sections lfor transmitting` a plurality of carrier currents forming a plurality of communication Channels on said line sections, two-way repeating means between said line sectionsffor all said channels, a circuit, means for connecting said circuit'with either line section, means in said circuit for selecting` a desired channel on the line section, a transmitter and a receiver, and means for connecting said transmitter and said receiver with said circuit;

15. In a carrier-wave transmission system, two line sections'for transmitting a plurality of carrier waves, each modnlated in accordance with signals, repeating means between said line Sections for repeating'said modulated waves, a transmitter and a receiver assocated with said're'peating means, and connections for utilizing said transmitter and receiver for transmitting or receiving 'a modulated wave over either said line section.

16. In a carrie'r-wave transmission system, two line Sections for transmitting a plurality of carrier Waves, each modula-ted inV accordance with signals, repeating means between said line Sections for repeating said modli.- Satrid Waves a transmitter and a receiver a.

sociated with said repeating means, and connections for utilizing said transmitter and receiver for transmitting or receiving a modulatcd wave over either said line section while said repeating is in progress.

17. In a signaling system, lines for transmit-ting currents of different frequencies, a transmitter and a receiver for transmitting and for recciving currents of any of said frequencies.V means for connecting said transmitter or said receiver to any one of said lines, and a selective circuit interposed between the transmitter or receiver and the line to which connection is made for permitting the passage of currents of a desired frequency between said transmitter or re ceiver and the connected line.

18. In a signaling system, a line terminating in coupling line windings and an artificial circuit for balancing said line, a circuit symmetrically associated with said line windings, a loading impedance normally across said circuit, a transmitter and a receiver and a single means for simultanc'ously removing said loading impedance from said circuit and inserting said transmitter or said receiver into said circuit.

19. In a carrier current signaling system, a line, coils having their windings in series in said line, a net to balance said line, a repeater having an input circuit bridged across the mid-points of said coils, a second repeater having an output circuit including windings inductively associated with certain of said first-mentioned windings, a circuit including` windings inductively associated with the others of said first-mentioned windings, and carrier current receiving and transmitting means arranged for connection to said last-mentioned circuit.

20. In a signaling system, a line having coupling-coil line windings and a net circuit for balancing said line, acircuit symmetrically associated with said line windings, a loading impedance normally in said circuit, signahng means f'or said line, and single means for simultaneously c'onnecting said signaling means to said circuit and removing said loading impedance from said circuit.

21. In a signaling system, lines for the simultaneous transmission of currents representingv a plurality of signals,rrepeating means between said lines for repeating all said currents, and monitoring means' associated with the repeating means for registering any one of said signals to the exclusion of other signals.

22; In a signaling system, lines for the simultaneous transmission of currents representing' a plurality of signals, repeating means between said lines for repeating all said currents, and monitoring means associated with :the re-peating means for registering any of vsaid ,signajll to the exclusiop. of

other` sign'als'an'd for transinitting signals over said lines.

.23. In .a signaling systen'r, lines for 'the siinultaneous transmissio'n of cnrrents representing a plurality of `messages, a repeating' station through which said enr'rents are silnultaneously repeated between ,said lines, and n'ionitor'ing means at the rep'eating sta` tion for registering a desi'red one 'of' said messages during its 'repetition '24. In a signaling system, lines 'for the siniultaneous transinission of currents representing a `phn'ality of finessages, a repeating station Ithroi'zgh which said currents are siinultaneonsly repeated between said lines, and "monitoring Ineans'at'-tl1e Vrepeating station for registering' 'a desired one of said messages during its repetition and fo'r transmitting over said lines.

25. In alsignaling system, lines for the simultaneous transn'lission of currents in both direetious 'representiirgI a vplurality of messages in each direction, a repeating station through which said eurrents arovsiinnl- Etaneously repeated in both directions he- 4tween said lines, `n1onitoring means at said repeating station for 'registering any one of said messages, and means for selectively transmitting cnrrents representi'ng any one of said Vmessages` froin said lines to said monitoring means.

26.111 a signaling system, lines for the simultalieous transinission of eurrents in hoth direetions i'epres'enting a plurality of =messages in'each direction, az'repeating station'through which 'said eurrents are. simultaneouslyfepeated in 'both direetions hetween'said lines, ``monitoring ;means at said 'repe'atin'g station for -registering any one'of said messages andfortransmitting overisaid lines,-and 'means for selectively transmitting 'eurrents representiiig :any-one ofsaid messageg' between said linesand said monitoringmeans.

`v27.'l5ijline'for the transmission of enrrents of'differ'ent frequencies, a generator for'said currents, filtering circuits each for transmitting `current of av given said frequency, and switching 'means for connecting said generator to said line through any one of said filtering circuits.

28. A line for the transmission of yoieemodulated current Waves,`a transmittercand a receiver for said Waves, band filters 'for selectiyely transmitting a -said modulated wave,'and means for eonneot-ing said transn'iitter and said receiver to said line through any oneI of said filters.

29. 'A line for transmittinga plurality of modnlated current Waves simultaneorisly, a filter for eachsaid wave, atransmitter and a receiver for said modulated Waves, and means for connecting said transmitter and said? reoeiyer "to 'said line through any iected one :of-*said filters.

*30. A 'line for the transins'sion of modulated 'current Waves =of different frequenci'es, a transinitter and a 'receivcr for allfsaid Waves, a isele'ctive path for each said modulated wave, and means for assooiating said transinitter and 'said receiver withsaid line through any one of said selective pa'rhs.

31. JIn a telephone system, lines for the siinultaneous transm'ission of currents representing a plurality of conversations, repeating means hetween'said lines for repeating 'said cn'rrents, and monitoring means for receiving currents 'representing any one of said eonversations land for transinitting eur-- rents over said lines.

82. In a tc'lephone system, 'lines for the Siniultaneous transmission of currents representing a plnrality 'of conversations, `re- 1` eating nieans between said 'lines for repeatin'g said eurrents, a receiver, and means for selec'tino` 'the currents representing any 'single said con'versation and for t iansmitting thein 'between'said lines and said re'ceiver.

33. A, signali'ng system coinprisinga line for thetransl'ni'ssion of cu'rrentwaves of differei'rtlfrequencies, a receiver for all 'said Waves, means 'included `in` said *reoeiver for changing the frequencies of all said Waves, a seleetive path for each of said Waves, and means for connecting said receiyer with said Vline through any oneoffsaid seleotive paths.

'34. A signaling system comprising a plurality of linesfor the transmission of earri'er wavesof different 'frequenoi'es, a receiver'for allsaidflwave's, a jseleotive Ipath 'for each of said 'waye's .and means'for connecting fsaid receiyer with Ianyone of "said lines through 'any'one'ofsaid'selectiye*paths f 35., :A trans'niision cirenit, 'totr'ans- *mit nie ges *thereoyen winonitoring 'operators fci'rcuitfforf supervising 'the messages Itransmitted oversaid transmis'sion cirouit, said rno`nitoringV oircni't comprising 'a sourcel 'of iscillations 'adapted to 'supply Carrie:` 'osoil'lations of jdiffe'rent frequencies, fr'e- `queney''selective devices,"and"means 'for asso- `eiating`a`ny one ofsaidf'sel'ective devices Iwith said source.

36. A transmission eirouit,=`rneans to transmit messages fthereover, a monitoring operatorls Circuit for vsupervising the messages transmitted over said transmission `oirouit, 'said yn'ionitori'ng circuit ``co'rnprising a source of o'scillations adapted to supply earrier Voscillations of different freqneneie's, cord eircuits V including devices selective `of V`said 'different frequencies, ,and means for"associjat- 'ing'anyone of said cord circuits with'said source. I

r 37; 'A'transmission circuit,'ineanstto transmit niessa eslthereover,amnitoring operators feireuit'for supervising the inessages `'tifal'rsnriitted 'dyfer said transmission circuit, 'said monitoring circnit comprising *af receiv- 'ing t'circnit including an 1 f'ar'nplifie'r "and 'detector, cord circuits including devices selective of different frequencies, and means for associating any one of said cord circuits With said receiving circuit.

38. A. line for transinssion of Voice modulated current Waves, a transmitter and a receiver for said Waves, a plurality of cord circuits having band filters for selectively transmitting a said modulated wave and means for connecting said transmitter and receiver with any one of said cord circuits.

39. A telephone system comprising a plurality of cord circuits each provided With selective means, each of said selective means adapted to pass a different range of frequencies than the others and a plurality of lines to any one of which any one of said Circuit-s is adapted to be connected.

40. A signal system comprising a pluralit-y of lines having a plurality of carrier current channels superimposed upon one another on each line, means at an intermediate station for connect-mg` said lines and supervising means at said station.

Lil. A signal system comprising a plu rality of lines, each adapted to transmit a plnrality of corresponding` different frequency carrier currents, means at an intermediate station for connecting said lines and means at said station for supervising` transmission over any carrier frequency channel.

4:2. .A carrier current system for connecting` a plu 'ality of terminal stations through an intermediate station comprising an intermediate station, a plurality of lines between said terminal stations and said intermediate station, means associated with each line for pern'iitting a plurality of different frequency 'arrier currents to be transmitted thereover, means at said intermediate station for interconnecting said lines and supervising' means at said intermediate station.

43. A carrier current system for connecting,` a plurality of terminal stations through an intern'iediate station comprising` an intermediate station, a plurality of lines between the terminal stations and the intermediate station, means 'associated With each line for permitting a plurality of different frequency carrier currents to be transmitted thereover, means at the intermediate station for inter- ('onnecting said lines, and means at said intermediate station for supervising transmission over any carrier frequency channel individually.

In witness Whereof, I hereunto Subscribe niy name this Seventh day of August A. D., 1919.

BURTON VV. KENDALL. 

